The present invention relates generally to charging devices and particularly to charging devices which produce a negative corona for use in an electrostatographic printing apparatus.
In today's electrostatographic printing apparatus a photoconductive insulating member is uniformly charged and subsequently exposed to a light image either by direct exposure through a light lens system or to a laser beam which is electronically controlled to expose the member in non-image areas. The electrostatic latent image so formed is made visible by developing with toner particles to form a powder image on the photoconductive insulating layer which may be subsequently transferred to a copy sheet and permanently affixed thereto by heat and pressure. Following transfer of the toner image, the photoconductive insulating layer may be discharged and cleaned of residual toner for the next imaging cycle.
Various types of charging devices have been used to charge or pre-charge photoconductive insulating layers. A recently developed corona charging device is described in U.S. Pat. No. 4,086,650 to Davis et al., commonly referred to in the art as a dicorotron wherein the corona discharge electrode is coated with a relatively thick dielectric material such as glass so as to substantially prevent the flow of direct current there through. This device has the advantage of providing a uniform negative charge to the photoreceptor and therefore may be used in conjunction with the multilayered electroconductive imaging photoreceptors described in U.S. Pat. No. 4,265,990 to Stolka et al. Briefly, these photoreceptors comprise at least two electrically operative layers, a photogenerating layer or a charge generating layer and a charge transport layer which are typically applied to the conductive layer. For further details of such a layer, attention is directed to the aforementioned patent.
As described in some recent patents, certain difficulties have been observed when using corona charging devices that produce a negative corona. It is believed that various nitrogen oxide species are produced by the corona and that these nitrogen oxide species are adsorbed by solid surfaces. In particular, it is believed that these oxide species are adsorbed by the conductive shield as well as the housing of the corona generating device. The shield may in principle be made from any conductor but is typically made from aluminum and the housing may be made from any of a number of structural plastics such as a glass filled polycarbonate. This adsorption of nitrogen oxide species occurs despite the fact that during operation the corona generating device may be provided with a directed air flow to remove the nitrogen oxide species as well as to remove ozone. In fact, during the process of collecting ozone the air flow may direct the nitrogen oxide species to an affected area of the charging device or even some other machine part. It has also been found that after such exposure when a machine is turned off for extended periods of idleness that the adsorbed nitrogen oxide species gradually are desorbed, that is the adsorption is a physically reversible process. It should be understood that the adsorbed and desorbed species are both nitrogenous but not necessarily the same, i.e., there may be conversion of NO.sub.2 to HNO.sub.3. Then, when the operation of the machine is resumed, a copy quality defect is observed in the copies produced in that a line image deletion or lower density image is formed across the width of the photoreceptor at that portion of its surface which was at rest opposite the corona generating device during the period of idleness. While the mechanism of the interaction of the desorbed nitrogen oxide species and the photoreceptor layers is not fully understood, it is believed that they in some way interact with the surface of the photoreceptor increasing the lateral conductivity so that it cannot retain a charge in image fashion to be subsequently developed with toner. This basically causes narrow line images to blur or to wash out and not be developed as a toner image. This defect has been observed with conventional selenium photoreceptors which generally comprise a conductive drum substrate having a thin layer of selenium or alloy thereof vacuum deposited on its surface as the imaging surface as well as with the multilayered electroconductive imaging photoreceptors described in the aforementioned U.S. Pat. No. 4,265,990.
Furthermore, with prolonged exposure of the photoreceptor to the desorbing nitrogen oxide species during extended periods of idleness the severity of the line defect or line spreading increases. While the mechanism is not fully understood it has been observed that even after a relatively short exposure period of time, 15 minutes, and a period of idleness of, say, several hours, a mild line defect and concurrent image deletion may be perceived. During the initial stage of exposure of the photoreceptor to the desorbing nitrogen oxide species, it is possible to rejuvenate the photoreceptor by washing with alcohol since reaction between the photoreceptor and the nitrogen oxide species is purely at the surface. However, after a prolonged exposure period of time the reaction tends to penetrate the photoreceptor surface layer and cannot be washed off with the solvent. Thus, for example, the problem is perceived after a machine has been operated for about 10,000 copies, rested overnight and when the operator activates the machine the following morning, the line deletion defect will appear. As indicated above the defect is reversible to some degree by a rest period. However, the period involved may be of the order of several days which to an operator is objectionable.
Similar difficulties are encountered in a corotron with a negative potential applied. Attempts to solve that problem by coating the corotron with a thin conductive dry film of aluminum hydroxide containing conductive particles are described in my U.S. Pat. No. 4,837,658 which is hereby incorporated by reference herein.
Several recent patents are directed to approaches to minimizing or solving these difficulties. In particular, U.S. Pat. No. 4,585,320 to Altavela et al. proposes plating the elements capable of adsorbing nitrogen oxide species with a thin layer of lead. U.S. Pat. No. 4,585,323 to Ewing et al. suggests the use of a continuous thin layer of a paint containing a reactive metal such as nickel, lead, copper, silver and zinc on the surfaces which adsorb the nitrogen oxide species. My own U.S. Pat. No. 4,585,322 provides an alkali metal silicate coating on the elements capable of adsorbing and neutralizing the nitrogen oxide species. Further, my own U.S. Pat. Nos. 4,646,196 and 4,920,266 teach coating the elements capable of adsorbing and neutralizing the nitrogen oxide species with a dry film of aluminum hydroxide or a dry film of aluminum hydroxide containing graphite and powdered nickel respectively.
All of the above coatings are capable of minimizing or solving the problem to varying degrees for varying periods of time. The coatings described in the last two mentioned of my prior patents are effective in minimizing the problem in the smaller, slower low volume machines. However, difficulties are experienced in the larger, much faster higher volume machines, wherein, it is desired that the corona charging devices have a life in excess of a million prints if not to the end of the functional life of the machine. Accordingly, corona generating devices which function for 30,000 to 40,000 prints such as those described in the last two aforementioned patents which have films of aluminum hydroxide provided on the corona generating device are consumed too quickly in the higher speed, higher volume product.
While not wishing to be bound to any particular theory, it is believed that this failure occurs by the conversion of NO.sub.x effluents and water vapor to HNO.sub.3 by products on the coatings surface during the operation of the corona generating device. It is also believed that the coatings provided in the above enumerated patents are consumed by the nitric acid like effluents over time thereby prematurely degrading it's performance and leading to the deletion difficulties.
It is further believed that the grain boundary is the most chemically reactive region in a given metal's micro structure which would provide the active sites for reaction of the NO.sub.x type of effluents and water vapor to HNO.sub.3.